1. Field of the Invention
The present invention relates to a method for making composite mullite/cordierite ceramics used as integrated circuit (IC) substrate materials by way of example.
2. Prior Art
Mullite has now attracted attention as IC substrate materials for fast elements, since its strength is close to that of alumina heretofore well-available to this end and its coefficient of thermal expansion is close to that of silicon. Problems with mullite, however, are that its coefficient of thermal expansion is still larger than that of silicon and its dielectric constant is high.
Cordierite, on the other hand, excels in thermal shock resistance, heat resistance and chemical stability. As already pointed out, however, problems with this material are that its mechanical strength is relatively insufficient and its coefficient of thermal expansion is much lower than that of silicon.
Set out in Table 1 are various physical properties, typically the coefficient of thermal expansion .alpha. (10.sup.-6 /.degree.C.) (measured at 25.degree.-800.degree. C.), dielectric constant .epsilon. (measured at 25.degree. C. and 1 MHz) and flexural strength .sigma. (MPa) of alumina, mullite and cordierite.
TABLE 1 ______________________________________ Physical Properties Alumina Mullite Cordierite Silicon ______________________________________ .alpha.* 8.1 5.6 1.5 3-4 .epsilon.* 9.5 6.6 5.0 .sigma.* 350 270 245 ______________________________________ *.alpha. Coefficient of thermal expansion (10.sup.-6 /.degree.C.) (measured at 25-800.degree. C.). *.epsilon. Dielectric constant (measured at 25.degree. C. and 1 MHz). *.sigma. Flexural strength (MPa).
In view of the foregoing, it has been attempted to form composite mullite/cordierite ceramics into substrate materials for LSIs, etc.
For instance, a ceramic substrate consisting substantially of 0.5-5.0% by weight of MgO and 95.0-99.5% by weight of Al.sub.2 O.sub.3 +SiO.sub.2, with the weight ratio of Al.sub.2 O.sub.3 to SiO.sub.2 being in the range of 50:50 to 80:20, has been proposed (Japanese Patent Publication No. 61-15532, and "Yogyo Kyokaishi", 95(10), pp. 1037-1039, 1987).
However, the method for making composite mullite/cordierite ceramics by powder mixing, set forth in these publications, poses problems that the resulting sintered compacts are so uneven in microstructure that their physical properties vary largely with a drop of their strength.
In particular, the content of MgO exceeding 5 wt. % causes an increase in the amount of spinel formed, offering problems that the coefficient of thermal expansion and dielectric constant increase. A certain limitation is placed on the amount of cordierite to be incorporated as well. For instance, any sintered compact having a dielectric constant of 6.5 or lower can never be obtained. Nor are sintered compacts rich in mechanical strength obtainable, because any sintering density exceeding 2.5 g/cm.sup.3 cannot be obtained at a relatively low sintering temperature, say, 1450.degree. C. In addition, the coefficient of thermal expansion achieved is relatively high, say, 3.8-3.9.times.10.sup.-6 /.degree.C. at best. Thus, some limits are imposed on improving the properties of substrate materials.
In another effort heretofore made (see "Am. Ceram. Soc. Bull.", Vol. 63, No. 5, 705 (1984), mullite powders are mixed with coerdierite glass powders to make composite mullite/cordierite ceramics. A problem with this cordierite-glass-adding technique, however, is that no sufficient sintering takes place, failing to give densified sinterings.
For instance, even sintering of a mixture of mullite powders with cordierite glass powder at a mixing ratio of 80:20-60:40 and at a temperature in the range of 1450.degree.-1455.degree. C. gives a density of barely about 60-72% of theoretical density.
In yet another effort, composite mullite/cordierite ceramics are produced by an alkoxide technique (see "Nihon Ceramics Kyokai Gakujutsu Ronbunshi", Vol. 96, No. 6, 659(1988)). Problems with this technique, however, are that the starting material is costly; the remaining glass phase causes a drop of strength; and any intimately sintered product cannot be obtained due to carbon residues.